The deployment of high speed data services over all-coax and Hybrid Fiber-Coax (HFC) has always been anticipated to follow a straightforward implementation strategy. However, recent industry experience deploying Cable Modem Termination Systems (CMTSs) has uncovered an unforeseen and challenging system engineering issue when deploying new services over HFC systems. The problem is a mismatch in CMTS upstream "ports" to the large number of return path "ports" on HFC systems. Adding more CMTS upstream ports places an operator in an undesirable up front capitalization situation as the additional upstream costs are well beyond the revenue stream during initial sparse deployment.
In general, there are two engineering and business rules to follow when deploying high speed data services on all coax plants: firstly, the new service must be available for any subscriber in the head-end serving area, typically a city or collection of adjoining towns, and secondly, the business model for deployment must incrementally add high-speed data CMTS equipment in track with subscriber demand (capacity) and the associated incremental gains in increased revenue stream.
All coax cable television distribution plants are well suited to the deployment of high speed data services: there are one to few downstream distribution coax trunks leaving the head-end and there are one to few upstream coax trunks entering the head-end. For initial deployment scenarios, a single downstream transmit channel (e.g., single 64 QAM 30 Mbps digital data channel) can service the entire all-coax cable plant by distributing the same downstream signal to all downstream trunks. Similarly, one or more CMTS upstream channels can share the same CMTS upstream port and that port can be coupled to more than one upstream trunk via the use of an RF combiner. One or more upstream data channels can be supported by the CMTS equipment, each separated by frequency. An upstream port is the F-connector which makes the 5-42 Mhz upstream spectrum available to the CMTS equipment.
The use of an RF combiner in the upstream to combine several trunks into one port is of limited use due to the known noise funneling problem (i.e., combining return trunks into a single upstream return port raises the noise floor at the port). The rise in the noise floor is a combination of both background system thermal noise and of externally generated ingress noise. These noise sources collectively form the impairment noise that must be overcome by the upstream data channel for any interactive service: high speed data services, impulse pay per view, etc. The number of upstream trunks that may combined is chiefly limited by the noise characteristics of the return plant. Some all-coax return plant trunks may be noisier than others.
Initial deployment of high speed data services on all coax plants can typically be accomplished using one CMTS for the entire plant. Existing CMTS equipment today come in one of two scalability architectures: "fixed" scale configuration with one downstream port with only one upstream port, or "flexible" scale configuration with one or more downstream ports with one or more upstream ports. Incremental growth to meet new subscriber demand or capacity is different for fixed versus flexible architecture.
At some point in the growth of service deployment, more downstream capacity will be required to meet subscriber demand. In a fixed scale CMTS, a new downstream channel is required for every upstream channel added and vice versa regardless of whether the downstream or upstream channel capacity has been filled by demand. In a flexible scale CMTS, the relationship of the downstream channels to the upstream channels within a single CMTS box are separately scalable, allowing the addition of downstream or upstream channels to follow subscriber demand. In addition, this flexible scale-ability allows for capital expenditures to more closely match revenue growth, and also allows for noise impairment to be better controlled by use of more upstream ports per downstream channel. This latter point is very important in that the cable operator has much more flexibility in managing the recombination of upstream trunks and subsequent noise funneling issues.
When the downstream channel capacity has been exceeded and not enough RF spectrum is available in the cable plant, the operator has the option of upgrading the plant to HFC. The upgrade to HFC will produce more downstream trunks and more upstream trunks. If previous CMTS purchases matched capacity and revenue growth, there is a likelihood that the existing installed CMTS equipment will match the newly available trunks and subsequent ports. Note that in this incremental HFC upgrade scenario, the cable operator has the option to do upgrades only where high-speed data capacity is needed (i.e., where the active subscribers and revenue is coming from). Upgrading the entire plant to HFC is not required.
The ideal world for high-speed data service deployment, business, and growth is to upgrade to HFC only after having established a revenue stream from high-speed data penetration. Since the world is not ideal, new high-speed data services must be deployed on existing all HFC plants. The issue that arises is the matching of CMTS upstream ports to the large number of cable plant upstream ports. The number of return ports is a direct function of node size, the smaller the node, the more ports. The ability to recombine upstream trunks is directly influenced by thermal noise issues of return path lasers and by ingress noise management.
The following example illustrates the return port abundance problem. Assume a small 20K House Holds Passed (HHP) plant is converted to all HFC with a node size of 500 HHP. This yields forty (40) separate returns. Assume that Fabre-Perot (FP) lasers have been used for the upstream returns based on their affordability. FP lasers allow a recombination of four to one (4:1), that is four upstream trunks can be recombined into one upstream port. This reduces the number of upstream ports to ten (10). These ten ports must be support by CMTS equipment.
With fixed scale CMTS equipment ten boxes are required. Worst case economic impact would be that where one box might have supported the entire previous all coax plant, nine additional boxes are now required. With flexible scale CMTS equipment one box is required, provided it supports ten (10) upstream return ports. The one box might have supported the previous all-coax plant and just rolls over to support the new HFC plant. Capital may be needed to purchase additional upstream channel demodulator support for the CMTS. Smaller node sizes increases the number of upstream return trunks. In the above example, if the node size was 2000 HHP instead of 500 HHP, then the number of returns trunks would have been ten (10) not forty (40). Ten return trunks could be recombined into three upstream return ports.
The port mismatch problem gets worse with a larger systems. A typical 50K or 200K HHP system greatly multiplies the number of upstream return ports. In the above 20K HHP example, a system which is 200K HHP is ten (10) times the number of return trunks (i.e. 400). Recombination yields 100 return ports. This is a significant number of ports that must be supported by CMTS equipment.
In the above 200K HHP model with a 500 HHP node size, engineering for initial deployment of high-speed services is faced with a significant challenge of supporting 100 upstream return ports with CMTS equipment. Recall that service must be made available to the entire serving area. The worst subscriber support scenario would be one subscriber per upstream return port. The available revenue from 100 subscribers is not sufficient to purchase CMTS equipment with 100 upstream return channels. Note that in this scenario, one downstream data channel is sufficient to supply services to any subscriber in the serving area until such a time as when demand exceeds the capacity of that single channel.
Recombining return trunks at greater than four to one (4:1) causes noise funneling contribution and reduces the Carrier-to-Noise Ratio (CNR) below a 25 dB margin at the upstream return port. This ratio is being used by several cable operators. Converting the upstream lasers from FP to Direct Feed-Back (DFB) lasers allows the upstream return trunks to be recombined at a ratio of up to ten to one (10:1) which is attractive. If the plant currently has FP lasers, the cost differential to go to DFB is substantial and in most cases prohibitive.
High noise floor interrupts all upstream modulation schemes in an HFC plant. The ability to recombine upstream return trunks is limited by the lowest capable interactive service; for example, impulse pay per view, interactive twoway node management protocols, etc. The recombination problem affects more than just high-speed data services for Internet.
This invention provides a reverse path multiplexing solution that allows CMTS equipment to multiplex upstream returns by recombining data for many returns but not recombining the noise.